I think you’re confusing physical traits with skills. Physical traits you either have or not. Skills you develop. That’s not to say Ayton doesn’t possess skills, he does. His midrange jumper and hook are absolutely deadly. But he is reliant on people getting him the ball and effectively making himself available for those shots. He really can’t take anyone, even lumbering centers, off the dribble consistently. He still isn’t automatic sealing a defender and receiving a pass into the post (but as i said earlier he improved both those traits). And his three ball really isn’t a weapon any defender needs worry about.
Embiid doesn’t have an issue sealing and receiving passes. He can take a defender off the dribble. It’s what makes him a superstar. Markennan can do both too, but with much less strength. But he’s usually being guarded by smaller players, so it plays to his versatility.
I’m not saying Ayton couldn’t be a #2. I’m saying to be a true, high level #2 he would still have to either further hone his seal and receive and create contact or develop handles. He’s capable of both, but both require work and determination.